In the past years, magnetoelectronics and spintronics emerged as a very active and promising field of exploratory research and developed as a new paradigm of nanoelectronics. One of the success stories was the development of magnetic tunnel junctions (MTJs) with large tunneling magnetoresistance (TMR) ratios, the break-through being achieved in 2004 with room-temperature TMR values of about 230% in MgO-based junctions.